The primary function of hemoglobin is to transport oxygen in vertebrates. Hemoglobin in adult vertebrates has the tetrameric structure ?2-?2, and fetal hemoglobin has the structure ? 2-?2. There is considerable interest in the regulation of the expression of fetal hemoglobin, as reactivation of the ? globin gene can ameliorate the clinical consequences of sickle cell anemia and thalassemia. The embryonic timing of ? globin expression varies between species. Differences in the control regions of the two ? globin genes in higher primates, associated with different developmental timing, may shed light on the regulation of the fetal switch. To better understand the regulation of ? globin expression, the hemoglobin polypeptides of fetal, newborn and adult individuals of the New World monkey, Cebus apella, were investigated. We have used both ESI- and MALDI-MS, coupled with peptide mass mapping, as an analytical tool for analyzing these globin chains. This approach has allowed us to identify the predominant ? form expressed in fetal hemoglobin. In addition, we have identified and characterized a posttranslationally modified beta chain, a glutathione adduct.